Sandwich structures are extensively used in the aerospace industry as well as other applications and implementations where weight reduction is a significant factor. One conventional sandwich structure includes thin face sheet laminates and a honeycomb core. Other configurations include spaced stringer or ribbed configurations and even columnar core members oriented at right angles and secured to the outer face sheets. (U.S. Pat. No. 3,328,218). Many disadvantages exist, however, with respect to these prior art structures.
Honeycomb core structures have been shown to be susceptible to moisture intrusion because of the many open cells inherent in the honeycomb core. Non-visible face sheet damage is sufficient to create a path for moisture intrusion. When moisture, condensing on the outside of an aircraft wing skin fabricated with honeycomb core panels is forced into the honeycomb core by the pressure differential between the sandwich interior and the atmosphere during descent, the moisture becomes trapped creating corrosion, property degradation and eventually a significant weight penalty resulting in costly corrective action. Corrosion of aluminum honeycomb cores and debonding from repeated freeze/thaw cycles are other known problems.
Moreover, additional problems occur due to impact damage from accident, tool drop, hail strike, and the like. The honeycomb core may be crushed proximate the point of impact even though the face sheet suffers no visual damage. And, resulting core shear failure may extend over even a greater area. Such core damage may result in face sheet buckling and delamination under stress. Visual inspection of an aircraft wing may not show the underlying core damage which may significantly lower design tolerances.
Therefore, honeycomb core structures, although efficient because of a high strength to weight ratio, are not suitable for all applications. Similar problems are inherent in the other designs discussed above.
Closed cell foam core structures limit moisture intrusion and improve impact resistance; however, they suffer from low specific shear strength relative to honeycomb core materials. A structure including only columnar core members attached at right angles and secured to the outer face sheets may buckle under compression but more importantly such an arrangement also characteristically exhibits poor shear strength.
Therefore, these core materials do not offer the high specific strength needed for lightweight structures in conjunction with the impact resistance and low moisture intrusion features required for reliable use in primary load carrying members.